1. Field of the Invention
The present invention relates to a photoelectric conversion apparatus for performing logarithmic compression of light signal quantity into logarithmic output, and a photoelectric conversion apparatus making use of a phototransistor.
2. Related Background Art
There are conventional photoelectric conversion apparatus including those for providing a linear output signal against signal current quantity and those for providing a logarithmically compressed output signal in order to gain a wide dynamic range. A prior art example of such logarithmic compression type photoelectric conversion apparatus is one making use of a so-called logarithmic amplifier.
FIG. 1 shows an example of circuitry of the logarithmic amplifier. In FIG. 1, numeral 1 designates a photodiode, 2 an operational amplifier (op amp), 3 a diode, 4 a constant-potential input, and 5 an output terminal. When light is incident to the photodiode 1, a photocurrent according thereto flows into the negative input terminal of op amp 2. In the so-called logarithmic amplifier composed of the operational amplifier (op amp) 2, the diode 3, the constant-potential input 4, and the output terminal 5, the diode 3 is interposed between the output terminal 5 and the negative input side of an inverting input terminal. In such a coordinate system that light quantity/photocurrent is represented on the axis of abscissa while output potential at the output terminal is on the axis of ordinate, a light quantity produces a logarithmically compressed output with flow of a photocurrent to the diode 3. The wide dynamic range property can be achieved in this way.
In the above prior art example, however, one op amp has to be assigned to one light signal current. Therefore, the prior art example had such a drawback that when the prior art example was applied to the photoelectric conversion apparatus composed of an array of many photodiodes, the apparatus needed to have op amps in the same number as the number of pixels and this increased the area and the supply current of the apparatus.